Life on Mars:

March 22, 2002.

MARTIAN DARK SPOTS UNDER DEBATE - PHYSICAL OR BIOLOGICAL?

In the beginning of last year, "wild claims" appeared on several web sites considered as "dubious" or of the "lunatic fringe" kind. The "claims" were that the Mars Global Surveyor orbiter camera photographs of Mars show "vegetation" and "artificial glass tubes ."

It did not take me long to check it out. The photographs were authentic and unaltered, often with references to the original and official Mars Global Surveyor photograph publishing server at the Malin Space Science Center.

There were two kinds of images involved:

First, the images showing alleged "artificial glass tubes" did only show sand dunes stretched at the bottom of canyons. The wind created these. They were not of glass, and not artificial. I could not even understand how the idea came out, because you only had to look at nearby pictures to see that fields of dunes has exactly that aspect, and the "tubular" aspect was obvious: the "tubes" were in canyons. Case closed for me!

The second type of images were much more interesting. They did show something resembling bushes or even trees, sometime with shadows. These "bushes" or "trees" or "giant fungi" do indeed look much like vegetation. But JPL had an other explanation: they were traces of defrosting, when the ice, water ice or carbon dioxide ice, in the permafrost heated in summer, it did not melt as a liquid but immediately and violently vaporized as a gas, because of the harsh conditions of Mars not allowing liquid water. I had already challenged the notion of the impossibility of liquid water, and many scientists agreed that liquid water on Mars is sometimes possible, but near the south pole where the "bushes" were shown, the JPL explanation made sense.

JPL stated it in the pictures captions: "attention, it looks like bushes but it is not bushes!" and in the page I devoted to these new images, I quite agreed that it seemed a good explanation, unless new data challenge it in the future. Why shout "vegetation!" if there is a more trivial and economical explanation? I was already convinced by the results of the Viking experiments combined with our better knowledge of Mars that there is life on Mars, very probably in the form of small microbes, but the "bushes" did not convince me and the "glass tubes" fantaisies pointed out that things seen from space on another planet are not necessarily to be taken for granted.

Then, the wide audience vulgarization web site space.com run a story according to which the famous Science Fiction writer Arthur C. Clarke of "2001" fame stated in a interview that these "bushes" are indeed the proof that there is vegetation on Mars. Unfortunately, the interview was removed from the space.com web site. How did I know? Well, not only because people said it was there, but because their site search still had the reference to the missing page, including the first lines. Search indexes are not necessarily updated every time a page is deleted of course, but I found this rather funny. Because Sir Clarke also mentioned the infamous "glass pipes," I thought that Science Fiction writers are definitely not scientists and have fantaisies of their own. If Sir Clarke had looked at the JPL comment of the "bushes" images, maybe he would have agreed with their explanation.

But then again, space.com and other similar sites, marsnews.com, marsdaily.com and other announced that "a team of Hungarian scientists has discovered the proof that there is vegetation on Mars in the MGS images." The Hungarian team has examined the southern spots in detail and reported that the spots appear in late winter and then disappear by summer. I wondered if they forgot to read the JPL explanation, or if they had found new reason to challenge it, but none of the news items was in-depth enough to find out. So the "bushes" were still defrosting traces to me, and their seasonal changes are not at odds at all with this trivial interpretation.

Enter now the European Space Agency scientists. They wonder if the Hungarian team may be right.

If they think so, they will have a look with ESA's planned Mars Express, scheduled to orbit the planet beginning in late 2003.

Luckily, new work has now indicated why scientists did not agree with the physical explanation and favor the biological explanation instead: the spots appear first at the margins of the dune fields but rarely on the dune ridges. Their location (which is independent of the elevation of the land) and shape (which is circular on flat surfaces but elongated on slopes) seems to be at odds with a physical explanation alone.

Better information about the "hungarian scientist" is now available: András Horváth, Tibor Gánti and Eörs Szathmáry from the Planetarium and the Institute for Advanced Study, Budapest, suggested the spots could be colonies of Martian microbes which wax and wane with the season. They hypothesize the spots are colonies of photosynthetic Martian microorganisms, which over-winter beneath the ice cap. As the Sun returns to the pole during early spring, light penetrates the ice, the microorganisms photosynthesize and heat their immediate surroundings.

A pocket of water, which would normally evaporate instantly in the thin Martian atmosphere, is trapped around them by the overlying ice. As this ice layer thins, the microorganisms show through in the color gray. When it has completely melted, they rapidly desiccate and turn black. This explains why many dark dune spots have a black center surrounded by a gray aureole, the Hungarian scientists said.